On the rate of cooling of aerated food grains

Abstract

Aeration is one of the most powerful tools available to the managers and handlers of stored grains. Irrespective of the initial temperature and moisture content of the grains, the biological and chemical phenomena that occur in the stored grains are significantly influenced by the wet bulb temperature of the air entering the grain. Several authors have developed mathematical expressions to estimate the rate at which aerated stored grains and other hygroscopic materials cool. They refer to ‘difficulties in comprehension’, remark that ‘results are difficult to interpret’ and ‘necessarily complicated’. The principal objective of this work is to clarify and quantify the factors that determine the rate of cooling aerated food grains. This is achieved by deriving an equation that arises as a direct consequence of the conservation of energy and moisture in aerated grains. It is found that the rate of cooling of aerated grains depends strongly on the sorption isotherm, a mathematical function that relates grain moisture content to the equilibrium relative humidity and temperature of their surrounding atmosphere. The integral heat of wetting, a function of the sorption isotherm, contributes to the effective specific heat of grains. However, its effects are typically neglected in analyses of the rate at which aerated grain cools because published sorption isotherms are insufficiently accurate for them to be included. It is proposed that a multidisciplinary approach be taken to measuring the psychrometric properties of food grains. • A new and simple equation is derived for the rate of cooling of aerated grains. • The equation highlights the factors that govern the rate of grain cooling. • Published sorption data lead to inconsistent values of the heat of sorption.